Dc operated positive ion accelerator and neutron generator having an externally available ground potential target

ABSTRACT

Sealed neutron generator containing an isotope of hydrogen having an ion source, an accelerator and a target containing a substance which reacts with impinging ions to produce neutrons maintained at ground potential and located to be available at the exterior of the generator.

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inventor Appl. Nov

Filed Patented Assignee tates atet Barney J. Carr, deceased late ofColorado Springs, C010. (by Elisabeth Helen Carr, administratrix) Apr.23, 1968 May 25, 197 1 Kaman Sciences Corporation Colorado Springs,Colo. Continuation-impart of application Ser. No. 363,064, Apr. 23,1964, now Patent No. 03,393,316, dated July 16, 1968.

DC OPERATED POSITIVE ION ACCELERATOR AND NEUTRON GENERATOR HAVING ANEXTERNALLY AVAILABLE GROUND POTENTIAL TARGET 12 Claims, 8 Drawing Figs.

US. Cl 250/845, 313/61 s1 Int.Cl G21g3/00,

G2lh [50] Field of Search 250/845, 83.6 (W);3l3/6l [56] References CitedUNlTED STATES PATENTS 3,246,191 4/1966 Frentrop 250/845 3,311,771 3/1967Wood et a1. 250/845 Primary Examiner-Ralph G. Nilson AssistantExaminerMorton .1. Frome Attorney-Anderson, Spangler and WymoreABSTRACT: Sealed neutron generator containing an isotope of hydrogenhaving an ion source, an accelerator and a target containing a substancewhich reacts with impinging ions to produce neutrons maintained atground potential and located to be available at the exterior of thegenerator,

PATENTED HAYZS |97l SHEET 4 [IF 4 IN VENTOR.

BAR NEY J. CARR. DECEASED,

ELISABETH HELEN CARR ADMINISTRATOR BY ATTORNEYS DC OPERATED {POSITIVEION ACCELERATOR AND NEUTRON GENERATOR HAVING AN EXTERNALLY AVAILABLEGROUND POTENTIAL TARGET This invention relates to neutron generatorsand, in particular, to a sealed neutron generator tube providing a highneutron flux and being of small dimensions. This application is acontinuation-in-part of my application Ser. No. 363,064 filed Apr. 23,1964, 1964, for SELF-RECTIFIED POSITIVE ION ACCELERATOR AND NEUTRONGENERATOR now U.S. Pat. No. 3,393,316 issued July 16, 1968.

Neutron generators of various types are well known in the art and, incarrying out various experimental test and analysis procedures in thefield of nuclear physics, a neutron source which is readily portable andprovides a high neutron output is highly desirable. Since many of thetest procedures and uses require rapid repetition of an intense shortpulse of neutrons, such a source of neutrons is highly desirable. Manyof the neutron generators available heretofore were too large for easymobility or were not capable of producing the desired neutron intensity.

The present invention contemplates and has for its principal object theprovision .of a highly efficient, simplified and relatively inexpensiveneutron generator wherein the T(d, n) He reaction or D(d, n) I-Iereaction is used to generate essentially monoenergic neutrons.

The present invention is concerned with providing an improved neutrongenerator as a source of high neutron output which avoids one or more ofthe disadvantages of prior an arrangements.

A further object of the present invention is to provide an improvedneutron generator tube in which the target thereof is at or near groundpotential.

A still further object of the present invention is to provideimprovements in the components, component arrangement and circuitry of apulsed neutron generator adapted for well logging and/or laboratory use.

Another important object of the present invention is the provision of aneutron generator including automatic pressure control for the generatortube to permit maximum efficiency of operation.

Still another important object of the present invention is the provisionof a novel means of supression of secondary electrons from the target ofthe generator tube.

Other objects are to improve and simplify the construction of a highlycompact neutron generator in a manner to permit fabrication atrelatively low manufacturing cost, the improved unit beingof ruggedconstruction and operable over a long service life with a minimum ofmaintenance.

For a better understanding of the present invention, together withfurther and other objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings and itsscope will be pointed out in the appended claims.

In the drawings:

FIG. I is a view in section of one embodiment of the neutron generatortube according to the present invention;

FIG. 2 is a view in section, with parts broken away to conserve space,of another embodiment of a neutron generator according to the invention;

FIG. 3 is a cross-sectional view of still another embodiment of theneutron generator tube of the invention;

FIG. 4 is a graphical representation of the anode current versuspressure within the neutron generator tube;

FIG. 5 illustrates a circuit diagram, partly schematic, of a completeneutron generator according to the present invention including a novelpressure control system;

FIG. 6 is a representation of the waveforms of the voltage and currentsupplied to the ion source at low and operating gas pressure;

FIG. 7 is a representation of the neutron output of a neutron generatorduring operation and target loading; and,

FIG. 3 is a schematic diagram of the circuit of FIG. 5 modified toprovide the application of direct current potentials to the neutrongenerator.

The production of fast neutrons is generally well known in the art, forexample, the production of neutrons by a T(d, n) He or D(d, n) Hereaction, but the devices for production of such have heretofore beeneither quite bulky and not adapted to portable use or have not permittedrapid repetitive pulsing which is highly desirable in certainapplications of laboratory and production uses. Both portability and thecapability of producing rapid high intensity pulses of neutrons arenecessary for certain tests and research work as, for example, some oilwell logging, some neutron reactor tests and some laboratory nuclearresearch.

Several neutron generators embodying the present invention are hereinillustrated and described and each operates effectively over longperiods of use to generate neutrons having an average neutron yield ofgreater than about 10 neutrons per second. The principles taught hereinare, of course, not limited to a generator of neutrons at one particularenergy level and, with suitable modifications, the illustratedgenerators will operate in an equally dependable manner to produceneutrons at other energy levels. For example, while each apparatusherein described involves a DT reaction, it is apparent that withcertain modifications of the ion source, target and voltages employed,either a T-D or a DD reaction could be obtained, the characteristics ofwhich are wellknown in the art.

With reference to FIG. 1 of the drawing, there is shown one embodimentof a neutron generator according to the invention which includes apositive ion accelerator generally referred to by reference numeral 10and a power supply 11, FIG. 5, from whence the accelerator l0 obtainsthe necessary voltages for operation.

The power supply 11 and accelerator tube 10 preferably mounted within aprotective housing, not shown, which may be filled with oil or gas underpressure or evacuated to protect the generator components from voltagebreakdown. Control and operation functions are preformed externally ofthe housing through suitable connections thereto.

As seen from FIG. 1, the positive ion accelerator tube 10 comprises asbasic component parts an ion source 12, an accelerator section 14 and atarget 16. Positive ions are produced in ion source 12 and areaccelerated in section 14 to target 16. The neutron generator includeson assembly housed in a glass and metal cylinder comprising acylindrical glass insulator 18, the ends of which have bonded thereto,by glass to metal seals, sleeves 20 and 22 having out-turned flanges 2kand 23. A metal disc 24 provides an end closure for the target end ofthe tube and is welded to flange 21 to provide a unitary structure. Thedisc 24 is provided with a centrally positioned aperture 25 receiving acup 26 having a target 28 attached to the closed end thereof. Radiallyof the central aperture 25, disc 24 is provided with several smallerapertures 30 and 32, and the like, with tubular extensions 31 and 33receiving insulators 34 and 36. Insulators 34 and 36 receive lead-ins 38and 40 in gastight relation and similar insulators receive leads 42 and4 3 in similar manner. At the other end of cylinder 18, a metal sleeveextension 46 is attached to flange 23 of sleeve 22 as by welding. Thesleeve extension 46 is closed at its outer end by disc 48 centrallyapertured to receive tube 50. Tube 10 is evacuated through tube 50 andsealed off as by pinch seal 52 when the processing of the tube iscompleted. Disc 48 is radially apertured at 53 to receive an extensionand insulator 54 through which conductor 94 passes.

A magnet 96 is mounted to surround the sleeve extension 46 and besubstantially coextensive therewith. A disc cathode 58, having a centralaperture 60, is positioned at the juncture of sleeves 22 and 46transverse of cylinder I8 and secured thereto in suitable manner as bywelding. A cylindrical cupshaped control electrode 62 is provided withan outwardly extending flange 64 at the open end which is attached tothe margin of cathode 58 again as by welding. The closed end ofelectrode 62 extends part way into the interior of cylinder I8 and isprovided with a central aperture 66 having the edges thereof flared topresent a smooth rounded surface toward the target 28. At the other endof cylinder 18, another cup-shaped electrode 68 is positioned with theopen end thereof toward the target. The electrode 68 is positioned withthe open end insulated from disc 24 by insulator means 70. The closedend of electrode 68 is provided with a central aperture 72 having theedges thereof flared toward the target to present a smooth surface inopposing relation to the smooth surface of electrode 62. Electrode 68 iselectrically connected to external lead-in 42 by means of conductor 74.Electrode 68 serves to suppress secondary electrons generated at thetarget and shapes the accelerating field so that a defocused spot ofions impinges on the target 28. Lead-ins 38, 40 and 44 are respectivelyconnected to reservoirs 76, 78 and 80 for the purpose to be explained asthe description progresses.

The ions source 12 of neutron generator tube 10 in addition to cathode58 is provided with cathode 82, both of which are preferably magnetic.Cathode 82 is of a disc configuration mounted within sleeve 46 andlocated adjacent disc 48. Centrally of cathode 82 is a cylindricalportion 83 concentrically positioned with respect to the disc and sleeve46 and provided with a recess 84 in the end thereof facing cathode 58.Intermediate cathodes 58 and 82 and supported by insulator 89 is anonmagnetic anode 88 of cylindrical configuration concentricallypositioned within sleeve 46 and in axial alignment with cathodes 58 and82. The disc portion of cathode 82 is provided with an aperture 90through which an electrical conductor 94 passes and is electricallyconnected to anode 88. Conductor 94 passes in gastight seal relationshipthrough insulator 54. Anode 88 is supported by insulator 89 attached tosleeve 46.

The reservoirs 76, 78 and 80 are preferably of zirconium or titanium andthe like, with reservoir 76 being loaded with tritium, 78 loaded withdeuterium and 80 being unloaded. The target 28 may be tritium loadedtitanium or zirconium supported on a copper backing being or forming apart of cup 26. Tritium or deuterium may be selectively introduced intothe interior of tube 10 by heating reservoirs 76 or 78 as by the passageof electrical current therethrough.

The magnetic field for ion source 12 is provided by a magnet 96surrounding sleeve 46. The ion source ionizes tritium and deuterium gasby means of a crossed electric and mag netic field supplied by magnet96, cathodes 58 and 62 and anode 88. Magnet 96 produces a magnetic fieldH parallel to the axis of anode 88. A field value, for example, of 400to 600 gauss, has been used with success. A positive voltage of aboutone to several thousand volts is applied to anode 88 from leadin 94, viaconductor 92, and anode 88 is operated positive with respect to thecathodes. The discharge produced in the ion source is the familiar PIG(Phillips ionization gauge) discharge; however, the dimensions, voltagesand the magnetic field strength chosen for use in the ion source permita large discharge current at pressures within the tube of from 10 to IDmm. of Hg as illustrated in the graph of FIG. 4.

The extracted ions leave the ion source 12 and pass through aperture 60of cathode 58 to be rapidly accelerated to target 28, which may betritium loaded titanium, for the production of neutrons at a highoutput. The target 28 and electrode 68 are driven negative in the rangeof up to about 200 kv. with respect to electrode 62 which is operatednegative with respect to anode 88 of the ion source. The ions move fromcathode 58 through opening 60 into the space between cathode 58 andelectrode 62 and exit through aperture 66 in electrode 62. The ions areaccelerated by electrode 68 through aperture 72 to drift to target 28where the ions interact with the target materials to produce neutrons.Electrode 68 serves to suppress secondary electrons generated at target28 and shapes the accelerating field so that a defocused spot of ionsimpinges thereon.

The opposing surfaces of electrodes 62 and 68 are provided withextremely smooth surfaces to avoid high voltage arcing therebetween. Theelectrode 68 furnishes an electric field distribution to cause the ionsto be defocused and reach target 28 in a spreadout fashion, but in amanner to avoid hitting electrodes 62, 68 or cup 26. When the ions,which are accelerated between electrodes 62 and 68 through apertures 66and 72,

strike target 28, they cause electrons to be released which, if

permitted to travel to the ion source 12, would produce a total 5current flow in the tube many times the ion current.

In order to reduce the total current flow in the tube, the tube in FIG.1 has electrode 68 electrically isolated from the rest of the structureby insulator 70. Electrode 68 is maintained more negative than thetarget to repell electrons from the accelerating gap between electrodes68 and 62.

It will be noted that target 28 at ground potential is planar and liesin a position to be completely accessible exteriorly of the tube.Further, the configuration permits the obtaining of high flux incontrast to a cylindrical target.

Referring again to FIG. 5, the power supply ill for tube I is seen tocomprise a step-up AC transformer 98, having a primary winding 100 forconnection to a suitable source of 60 cycle electrical alternatingcurrent or other low frequency current. The power for the ion source 12is supplied by the windings l04106 of the secondary winding 102 oftransformer 98. The voltage applied is at least 4 kv. peak to peak andthe current drain is on the order of about -20 ma. The ion source 12generates ions only during the positive halfcycle of the source voltagewhen source current flows as shown graphically in curves a, b and c ofFIG. 6. Voltage for target 28 furnished by windings 106-110 oftransformer 98. Electrode 68 is held negative with respect to target 28,which is at ground potential, during the time that beam current flows inthe tube.

Tube encloses a very clean evacuated chamber maintained at a high degreeof vacuum into which deuterium or tritium gas is furnished at a pressureof from between about 2X10 mm. Hg to about l 10 mm. Hg. The tube isprovided with reservoirs 76, 78 and 80 as shown in the form of a heatercoil of titanium or zirconium which is loaded with tritium or deuteriumin a manner known to the art. The tube is outgased and sealed off as atseal 52. For purposes of illustration, reservoir 78 is loaded withdeuterium, reservoir 76 is loaded with tritium and reservoir 80 is leftclean to act as a getter.

Target 28 may be preloaded with tritium or may be loaded and replenishedin situ, as well be described as the description progresses. The tritiumof the target is depleted during the production of neutrons and must bereplaced. The tritium is derived from reservoir 76 when the reservoir isheated by the passage of an electrical current therethrough. In likemanner, deuterium is introduced by reservoir 78 to be ionized andaccelerated to target 28. Deuterium entering the reaction and thatpumped into the walls of the ion source is replaced from reservoir 78.Nonreaction gases are gathered by reservoir 80.

The target current can conveniently be used to control the pressurewithin tube 10. Referring again to FIG. 5, a circuit to accomplish suchcontrol is illustrated. AC power is stepped-up in transformer I12 andrectified by diode 114 as a DC power supply for transistor amplifier116. A filter network 118 filters the rectified current and zener diodes120 provide voltage regulation. An input signal for amplifier 116 isderived from lead 108 of transformer 98 which is proportional to thetarget current. Since target current is a function of tube pressure, thesignal is also proportional to and changes as the gas pressure changeswithin the tube. The output of amplifier I I6 from the collector andemitter is a positive going and negative going DC signal. The triggerlevel on diodes I22 and 124 causes the forward or reverse windings 126or 128 of motor 130 to be selectively activated at a predeterminedsignal level from amplifier 116. Motor 130 is connected to drive thewiper arm of rheostat I32. Rheostat 132 is connected at one end toground potential through the secondary winding of transformer I34 and atthe other end through reservoir 78. Thus, the amount of deuterium gasreleased by reservoir 78 is controlled by the temperature produced bycurrent flow therein which is a function of the positioning of the wiperarm of rheostat I32. The wiper arm is positioned by motor I30 moving ina direction determined by the output of amplifier llll6, which is inturn determined by the input thereto from lead 108. The particular gaspressure to be maintained is determined by the trigger level of diodes122 and 124. The above control system adjusts the tube pressure for thenext operation of the accelerator and adjusts the pressure only duringthe operation of the tube.

The target 28 may be loaded initially and reloaded after the tube issealed by a novel process to be described. The target 28 comprises acopper disc having a thin layer of titanium evaporated thereon. Thetarget assembly 16, including reservoirs 76, 78 and 30, is assembledalong with the complete tube assembly, including ion source 12 andaccelerator 14. The tube is checked for leaks and placed on a vacuumsystem. The system is pumped down to on the order of less than about ll0 mm. Hg. Heat is applied externally of the tube to outgas same,raising the temperature to the neighborhood of about 415C whilemaintaining the pressure less than about 1X10 mm. Hg. The pumping andheating is continued until the pressure is less than about 1X10" mm. Hg.A loading reservoir, not shown, of tritium external of pinch tube 50 isheated to evolve tritium gas and deliver same into the interior of tubethrough tube 50. A relatively low voltage is applied to the tube of theorder of about to kv. between the target and cathode and the heating ofthe loading reservoir is continued until a discharge is obtained in thetube. The tube is operated under these conditions for about an hour at 2to 3 ma. beam current. The power is then removed from the tube and thetritium gas pressure is raised to about 100 mm. Hg. Reservoir 76 isheated by passing an electric current therethrough and then is allowedto cool to take up all of the tritium gas present in the tube to thusload reservoir 76. V

Deuterium gas is introduced into the tube from an external source, notshown, through pinch tube to a pressure of from about 10 to about 100mm. Hg. Reservoir 78 is heated by an electric current and then cooled totake up all of the deuterium present. The pinch tube 50 is then sealedoff as at 52.

The tube is placed into operation with the target being at a potentialon the order of about l00l70 kv. Reservoir 78 is heated to introducedeuterium gas and maintain a suitable gas pressure which may be, forexample, on the order of about 2X10 mm. Hg. The tube is operated underthese conditions for a period until the neutron output drops about 20percent due to depletion of tritium in target 28. An operating period ofabout 2 hours will normally produce this result. To reload the target,the operating voltage is then reduced to about 25-75 kv. at an increasedcurrent of 2 to 3 ma. Deuterium introduction is terminated and tritiumintroduction is started. The tube is operated at these conditions for asufficient length of time to reload the target, which will normally takeabout 12 to 15 minutes. The target 28 is thus reloaded withtritiumduring which neutron output is quite low. FIG. 7 represents aplot of neutron output of tube 10 versus time during nonnal operationand target reloading. After the target has been reloaded for a period oftime sufficient to replenish the depleted tritium in target 2%, thevoltage on the target is again raised to the operating voltage, l00l70kv., the tritium reservoir 76 is shut down and deuterium gas isintroduced from reservoir 78 to resume high neutron output. This processof operation and target reloading may be repeated as often as needed tomaintain the required neutron output.

In the embodiment of the invention illustrated in FIG. 1, the electroncurrent is essentially obviated and the tube is protected from excessiveelectron current by having electrode 68 at a more negative potentialthan the target 28, thus repelling electrons from the accelerating gapbetween electrodes 62 and 68. Also, as previously mentioned, withrespect to FIG. 5, electrode 6% being insulated from the target permitsthe use of beam current as a measurement of gas pressure within thetube. The arrangement of FIG. ll requires that the tube be operated,however, before the gas pressure in the tube can be regulated.

Referring now to FIG. 2, there is shown a neutron generator tube 10awith parts broken away, being identical to the righthand side of tube R0of FIG. 1. FIG. 2 has parts broken away to conserve space. In contrastto the arrangement of FIG. I, electrode 68a is electrically connnectedto disc 24 and is maintained at ground potential along with target 28.This construction is a much more rugged type of construction than thatof FIG. 1. An additional element in the form of an electron suppressionelectrode 140 of rod-shape is provided within the hollow wall portion ofelectrode 68a and connected to lead 142 insulated from disc 24 byinsulator 144. The suppression electrode 140 comprises a small diameterwire which serves as an anode of a small discharge device. The dischargedevice uses the target and interior of electrode 68a as the cathode.Electrode 140 may be quite small compared to the cross-sectional area ofthe wall portion of 68a, on the order of about 0.00l to about 0.005inches in diameter, to obtain a measure of gas pressure from about l0 to20 l0 mm. Hg. A potential of from 3 to 4 kv. is applied to anode 140from lead of transformer 98, FIG. 5. The discharge of the device islimited by a large resistor 146 in series therewith, making itessentially a constant current device. During the positive half-cycle ofthe power supply, the pressure in the tube is measured by measuring thevoltage drop across the resistor 146. During the negative half-cyclewhen beam current flows in tube 10, the anode compels electrons emittedfrom the target 28 as a result of ion impingement to be repelled andprevented from entering the acceleration gap existing between electrodes68a and 62.

Referring now to the embodiment of FIG. 3, the neutron generatorincludes a positive ion accelerator 10 and a power supply 111, FIG. 5,which supplies the operating voltages from a low frequency alternatingcurrent source, such as 60 cycle and the like. The positive ionaccelerator comprises an ion source 12, an accelerator section 14b and atarget section 16a. The neutron generator includes an assembly housed ina glass and metal cylinder including a cylindrical glass insulator 18,the ends of which have bonded thereto, by glass to metal seals, sleeves20 and 22 having out-turned flanges 21 and 23. A metal disc 24 providesan end closure for the target end of the tube and the disc is welded toflange 21 to provide a unitary structure. Disc 24 is provided with acentrally located opening 25 receiving a cup 26. A target 28 is mountedon the inside of the closed end of cup 26. The annular portion of disc24 around opening 25 is provided with a plurality of smaller openings30, 32 and the like receiving tubular extensions 31a, 33 or the like. Aconductor 38 passes into the interior accelerator 10 through extension33 and sealed thereto in gastight relation by insulator 36. Conductors40 and 44 are similarly provided for. At the end of cylinder 18 oppositedisc 24, a tubular metal sleeve extension 46 is secured to flange 23 ofsleeve by welding and the like. Sleeve 46 is closed at its outerextremity by disc 48. Disc 48 is centrally apertured to receive tube 50and apertured at 53 and 55 to receive an extension and insulators 54 and51 through which conductors 94 and 95 pass into the interior of thetube. Tube 50 provides the means through which the tube 10 is evacuated.After the processing of the tube is completed, it is sealed off by meansof a pinch seal as at 52.

Magnet 96 is mounted to surround the sleeve extension 46 and issubstantially coextensive therewith. A cathode 58 is positioned at thejuncture of sleeves 22 and 46 to be transverse of cylinder 18. Cathode58 is fastened to sleeve 46 as by welding and has a central opening 60therethrough. A eylindrical cup-shaped control electrode 62 has anoutwardly extending flange 64 at the open end. Flange 64 is secured tocathode 58 and flange 23 as by welding. The closed end of electrode 62extends into the interior of cylinder l8 and has an opening 66 centrallylocated in the closed end. The edges of opening 66 are flared to presenta smooth rounded surface toward target 28. A focus and gating electrode148 is positioned intermediate cathode 58 and electrode 62. Electrode148 is of tubular configuration and is supported on insulation members150 attached to cathode 58 to be in axial alignment with opening 60 incathode 5% and opening 66 in electrode 62. A lead-in conductor 152 isconnected to electrode 148 and passes through an opening 154 in cathode58, an opening 156 is insulator 89 and an opening in cathode 82 to passexteriorly of tube in gastight relation through insulator Sll. At theother end of cylinder l8, another cup-shaped electrode 68 is positionedwith the open end thereof toward the target. The electrode is mountedupon disc 24 by means of insulator 70. The closed end of electrode 68 isprovided with a central opening 72 having the edges thereof flaredtoward target 28 and presenting a smooth surface in opposing relation tothe smooth surface of electrode 62. The purpose of the opposing smoothsurfaces is to prevent arcing therebetween even when at great potentialdifferences. Electrode 68 is electrically connected to an externallead-in 42 by means of a conductor 74. Electrode 68 is normallymaintained at a negative potential with respect to target 28 and servesto suppress secondary electron emission from the target. Lead-ins 38; 40and 44 are connected respectively to reservoirs 76, 78 and 80, which maybe energized to control gas pressure.

Within the extension 46 of the ion source 12 and adjacent end closuredisc 48, a cathode 82. is positioned. Cathode 82 is of a discconfiguration and like cathode 58 is magnetic material. Cathode 82 isformed with a centrally located cylindrical portion 83 axially extendingtoward cathode 58. The end of the cylinder'83 toward cathode 58 isprovided with a recess 84. A tubular nonmagnetic anode 88 is positionedbetween cathodes 82 and 58 and supported by insulator 89 which isattached to sleeve 46 in a suitable manner. Lead-in conductor 94 passesthrough insulator 54 and opening 90 in cathode 82 to connect to anode88.

The reservoirs 76 and 78 may be tritium and deuterium loaded titanium orzirconium. Reservoir 80 is left unloaded to act as a getter. Target 28may comprise a copper backing with a layer of titanium or zirconium,loaded with tritium. Tritium or deuterium gas is introduced into theinterior of the tube 10 by passage of current through reservoirs 76 and78.

A magnetic field is provided by a magnet 96 surrounding sleeve 46 of theion source 12. Tritium and deuterium gas is ionized in an ion source 12by means of a crossed electric and magnetic field supplied by cathodes58 and 82, anode 88 and magnet 96. Anode 88 is operated positive withrespect to cathodes 58 and 82. The extracted ions leave the ion source12 and pass through aperture 60 of cathode 58 on their way to target 28.The ions leaving the source are controlled by a potential on electrode148 so that, in the region of this electrode, the direction and speed ofthe ion travel can be modifiedto either stop the ions or to cause themto follow optimum paths for focus characteristics on target 28. Apositive voltage some higher than that applied to anode 88 will shut offthe flow, and a voltage applied to electrode 140 which is negative withrespect to that applied to cathode 58 increases the efficiency of theextraction of ions from the ion source. The embodiment of FIG. 3includes a PlG pressure gauge 158 whose magnetic field is supplied bymagnet 160 which also suppresses secondary electrons from target 28.Gauge 158 operates on the same principle as ion source 12 and the anode162 is maintained positive with respect to the cathodes 164 which arealso the containing walls of the gauge. Voltage is applied to anode 162through lead 166, and the anode current of the gauge is a function oftube pressure and may be used for control purposes to gauge and regulategas pressure in the tube in a manner similar to that described withrespect to FIG. 5.

It will be appreciated that the several purposes have been accomplishedin providing a neutron generating device wherein the target is at groundpotential and is readily accessible for close proximity of target tothat being irradiated. Further, the neutron generator operates from analternating current source directly and obviates the necessity of costlyand cumbersome voltage rectifying devices.

Further, the embodiment of FIG. 2 provides an electron suppressionelectrode which also serves as an anode of a small discharge deviceusing the target and adjacent electrode as the cathode to provide animproved gauge for the control of gas pressure within the interior ofthe tube.

Referring now to FIG. 8, there is shown a circuit diagram similar tothat of FIG. 5 except there is provided a DC power supply. Diode M8, inconjunction with capacitor 170, provides a DC voltage such that anode88, the source anode, is approximately 7 kilovolts positive with respectto control electrode 62, the source body. Diode 172 and capacitor 174provide a DC voltage, making ion source l2 and control electrode 62positive with respect to electrode 68. This potential may vary fromabout I00 kilovolts to about 250 kilovolts depending on the size of theaccelerator tube. Diode 176 and capacitor 178 provide approximately Ikilovolt DC to render electrode 68 negative with respect to target 28for the suppression of secondary electrons. Otherwise, the circuit isquite similar in operation to that of FIG. 5.

While therehav'e been described what at present are considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention. It is aimed, therefore, inthe appended claims to cover all such changes and modifications whichfall within the true'spirit and scope of the invention.

It is claimed:

1. Apparatus for generating neutrons comprising in combination, anelongated generally cylindrical envelope defining a sealed chamberconfining a gaseous hydrogen isotope under low pressure, said chambercontaining an ion source, a target axially spaced from said sourceadapted to include a substance which reacts with impinging ions toproduce neutrons, means for selectively releasing an isotope of hydrogenwithin the chamber, accelerator means for accelerating and directingions from said source to said target, means for applying a DC electricalpotential to said ion source for production of ions and between said ionsource and said target to accelerate ions into neutron producingcollision with said target and means maintaining the target at groundpotential during neutron generation wherein the target is planar and ispositioned at one axial extremity of the cylindrical envelope formingone end thereof to provide a substantially uniform source of neutronsimmediately available externally of said envelope.

2. The apparatus as set forth in claim ll wherein the isotope ofhydrogen within the chamber is selected from the group consisting ofdeuterium and tritium gases.

3. Apparatus in accordance with claim l wherein the chamber furthercontains means to suppress secondary electron emission from the target.

4. Apparatus as in claim 3 wherein the means to suppress secondaryelectrons from the target includes a cylindrical electrode having anapertured wall at one end thereof.

5. Apparatus as in claim 4 wherein the cylindrical electrode is operatedat a negative potential with respect to the target during electronsuppression.

6. Apparatus for generating neutrons comprising in combination anelongated generally cylindrical envelope defining a sealed chamberconfining a gaseous hydrogen isotope under low pressure, said chambercontaining an ion source, a target axially spaced from said sourceadapted to include a substance which reacts with impinging ions toproduce neutrons, means for selectively releasing an isotope of hydrogenwithin the chamber, accelerator means for accelerating and directingions from said source to said target, means to suppress secondaryelectron emission from said target, means for applying a DC electricalpotential to said ion source between said ion source and said target toaccelerate ions into neutron producing collision with said target andmeans maintaining the target at ground potential during neutrongeneration wherein the target is planar and is positioned at one axialextremity of the cylindrical envelope forming one end thereof to providea substantially uniform source of neutrons immediately availableexternally of said envelope.

7. Apparatus as set forth in claim 6 wherein the isotope of hydrogenwithin the chamber is selected from the group consisting of deuteriumand tritium gases.

8. Apparatus as in 'claim 6 wherein the means to suppress the secondaryelectrons from the target includes a cylindrical electrode'having ahollow wall portion.

9. Apparatus as in claim 8 wherein the cylindrical electrode is operatedat a potential negative with respect to the target during electronsuppression.

10. Apparatus as in claim 9 including means for continuously evacuatingand maintaining said chamber under a high degree of vacuum.

11. Apparatus for generating neutrons comprising in combination, anelongated generally cylindrical envelope defining a sealed chamberconfining a gaseous hydrogen isotope under low pressure, said chambercontaining an ion source, a target axially spaced from said sourceadapted to include a substance which reacts with impinging ions toproduce neutrons, means for selectively releasing an isotope of hydrogenwithin the chamber, accelerator means for accelerating and directingions from said source to said target and means for applying a directcurrent potential between said ion source and said target to accelerateions into neutron producing collision with said target, wherein thechamber further contains means to suppress secondary electron emissionfrom the target, wherein the means to suppress secondary electrons fromthe target includes a cylindrical electrode having an apertured wall atone end thereof, wherein the electrode is maintained at the samepotential as the target and further includes an inner electrode of rodshape positioned within the cylindrical electrode, the cross-sectionalsize of the inner electrode being very much smaller than thecross-sectional area within the wall portion wherein the target isplanar and is positioned at one axial extremity of the cylindricalenvelope forming one end thereof to provide a substantially uniformsource of neutrons immediately available externally of said envelope.

12. Apparatus for generating neutrons comprising in combination anelongated generally cylindrical envelope defining a sealed chamberconfining a gaseous hydrogen isotope under low-pressure, said chambercontaining an ion source, a target axially spaced from said sourceadapted to include a substance which reacts with impinging ions toproduce neutrons, means for selectively releasing an isotope of hydrogenwithin the chamber, accelerator means for accelerating and directingions from said source to said target, means to suppress secondaryelectron emission from said target and means for applying a directcurrent potential between said ion source and said target to accelerateions into neutron producing collision with said target, wherein themeans to suppress the secondary electrons from the target includes acylindrical electrode having a hollow wall portion, wherein theelectrode is maintained at target potential and further includes aninner electrode of rod shape positioned within the cylindricalelectrode, the crosssectional size of the inner electrode being verymuch smaller than the cross-sectional area within the wall portionwherein the target is planar and is positioned at one axial extremity ofthe cylindrical envelope forming one end thereof to provide asubstantially uniform source of neutrons immediately availableexternally of said envelope.

1. Apparatus for generating neutrons comprising in combination, anelongated generally cylindrical envelope defining a sealed chamberconfining a gaseous hydrogen isotope under low pressure, said chambercontaining an ion source, a target axially spaced from said sourceadapted to include a substance which reacts with impinging ions toproduce neutrons, means for selectively releasing an isotope of hydrogenwithin the chamber, accelerator means for accelerating and directingions from said source to said target, means for applying a DC electricalpotential to said ion source for production of ions and between said ionsource and said target to accelerate ions into neutron producingcollision with said target and means maintaining the target at groundpotential during neutron generation wherein the target is planar and ispositioned at one axial extremity of the cylindrical envelope formingone end thereof to provide a substantially uniform source of neutronsimmediately available externally of said envelope.
 2. The apparatus asset forth in claim 1 wherein the isotope of hydrogen within the chamberis selected from the group consisting of deuterium and tritium gases. 3.Apparatus in accordance with claim 1 wherein the chamber furthercontains means to suppress secondary electron emission from the target.4. Apparatus as in claim 3 wherein the means to suppress secondaryelectrons from the target includes a cylindrical electrode having anapertured wall at one end thereof.
 5. Apparatus as in claim 4 whereinthe cylindrical electrode is operated at a negative potential withrespect to the target during electron suppression.
 6. Apparatus forgenerating neutrons comprising in combination an elongated generallycylindrical envelope defining a sealed chamber confining a gaseoushydrogen isotope under low pressure, said chamber containing an ionsource, a target axially spaced from said source adapted to include asubstance which reacts with impinging ions to produce neutrons, meansfor selectively releasing an isotope of hydrogen within the chamber,accelerator means for accelerating and directing ions from said sourceto said target, means to suppress secondary electron emission from saidtarget, means for applying a DC electrical potential to said ion sourcebetween said ion source and said target to accelerate ions into neutronproducing collision with said target and means maintaining the target atground potential during neutron generation wherein the target is planarand is positioned at one axial extremity of the cylindrical envelopeforming one end thereof to provide a substantially uniform source ofneutrons immediately available externally of said envelope.
 7. Apparatusas set forth in claim 6 wherein the isotope of hydrogen within thechamber is selected from the group consisting of deuterium and tritiumgases.
 8. Apparatus as in claim 6 wherein the means to suppress thesecondary electrons from the target includes a cylindrical electrodehaving a hollow wall portion.
 9. Apparatus as in claim 8 wherein thecylindrical electrode is operated at a potential negative with respectto the target during electron suppression.
 10. ApparaTus as in claim 9including means for continuously evacuating and maintaining said chamberunder a high degree of vacuum.
 11. Apparatus for generating neutronscomprising in combination, an elongated generally cylindrical envelopedefining a sealed chamber confining a gaseous hydrogen isotope under lowpressure, said chamber containing an ion source, a target axially spacedfrom said source adapted to include a substance which reacts withimpinging ions to produce neutrons, means for selectively releasing anisotope of hydrogen within the chamber, accelerator means foraccelerating and directing ions from said source to said target andmeans for applying a direct current potential between said ion sourceand said target to accelerate ions into neutron producing collision withsaid target, wherein the chamber further contains means to suppresssecondary electron emission from the target, wherein the means tosuppress secondary electrons from the target includes a cylindricalelectrode having an apertured wall at one end thereof, wherein theelectrode is maintained at the same potential as the target and furtherincludes an inner electrode of rod shape positioned within thecylindrical electrode, the cross-sectional size of the inner electrodebeing very much smaller than the cross-sectional area within the wallportion wherein the target is planar and is positioned at one axialextremity of the cylindrical envelope forming one end thereof to providea substantially uniform source of neutrons immediately availableexternally of said envelope.
 12. Apparatus for generating neutronscomprising in combination an elongated generally cylindrical envelopedefining a sealed chamber confining a gaseous hydrogen isotope underlow-pressure, said chamber containing an ion source, a target axiallyspaced from said source adapted to include a substance which reacts withimpinging ions to produce neutrons, means for selectively releasing anisotope of hydrogen within the chamber, accelerator means foraccelerating and directing ions from said source to said target, meansto suppress secondary electron emission from said target and means forapplying a direct current potential between said ion source and saidtarget to accelerate ions into neutron producing collision with saidtarget, wherein the means to suppress the secondary electrons from thetarget includes a cylindrical electrode having a hollow wall portion,wherein the electrode is maintained at target potential and furtherincludes an inner electrode of rod shape positioned within thecylindrical electrode, the cross-sectional size of the inner electrodebeing very much smaller than the cross-sectional area within the wallportion wherein the target is planar and is positioned at one axialextremity of the cylindrical envelope forming one end thereof to providea substantially uniform source of neutrons immediately availableexternally of said envelope.